At present an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) consists of evolved Node Bs (eNBs), and FIG. 1 illustrates the network architecture of the E-UTRAN, where an eNB functions as an access network and communicates with a User Equipment (UE) via an air interface. There are both a control plane connection and a user plane connection between the UE and the eNB.
Each UE attached to the network is served by a Mobility Management Entity (MME) which is referred to a serving MME of the UE, and the MME is connected with the eNB via an S1-MME interface which provides the UE with control plane services including mobility management and bearer management functions; and each UE attached to the network is served by a Serving Gateway (S-GW) which is referred to as a serving S-GW of the UE. The S-GW is connected with the eNB via an S1-U interface which providing the UE with user plane services, and user plane data of the UE are transmitted between the S-GW and the eNB over an S1-U bearer.
As there is a constantly growing demand of subscribers for a data service rate and a service capacity, the traditional scheme for the network with single-layer coverage by a macro eNB has not been able to satisfy the demand of the subscribers, so layered network deployment has been introduced in the 3rd Generation Partnership Project (3GPP), and FIG. 2 illustrates the network architecture of the layered network, where a macro eNB provides underlying coverage, and a low-power local eNB provides hotspot coverage; and there is a data/signaling interface (which can be a wired or wireless interface) between the local node and the macro eNB, and a UE can operate while being served by the macro eNB or the local eNB.
Since a cell controlled by the local eNB (e.g., a small cell) has such a small coverage area that there are a small number of UEs served by the cell, a UE connected with the local eNB tends to be provided with a higher quality of service, e.g., a higher service rate, a link with a higher quality, etc. Thus when a UE connected with the macro eNB enters the coverage area of the cell controlled by the local eNB, the UE can be transferred to the local eNB to obtain the service provided by the local eNB; and when the UE leaves away from the coverage area of the cell controlled by the local eNB, the UE needs to be transferred to a cell controlled by the macro eNB to keep wirelessly connected.
FIG. 3 illustrates a X2 handover process including the following operations in the existing Long Term Evolution (LTE):
Operation 301: A source eNB configures a UE for measurement, and the UE performs measurement according to received measurement configuration information;
Operation 302: The UE reports a measurement result to the source eNB to assist the source eNB in making a handover decision;
Operation 303: The source eNB makes a handover decision. If the source eNB decides to perform handover, then the process continues with subsequent operations;
Operation 304: The source eNB transmits a handover request message carrying handover preparation related information to a target eNB;
Operation 305: The target eNB performs admission control according to the received handover request message and configures a new bearer including a Signaling Radio Bearer (SRB), and Packet Data Convergence (PDCP), Radio Link Control (RLC), Media Access Control (MAC) and other entities. If the configuration succeeds, then the operation 306 will be performed;
Operation 306: The target eNB returns a handover request response message to the source eNB;
Operation 307: The source signals a received handover command to the UE in a Radio Resource Control (RRC) connection reconfiguration message and also stops data from being transmitted and received with the present eNB for the UE; and the UE stops data from being transmitted and received with the source eNB upon reception of the RRC reconfiguration message;
Operation 308: The source eNB transmits Serial Number (SN) state information of ongoing data transmission to the target eNB;
Operation 309: The UE initiates an uplink/downlink synchronization procedure to the target eNB, and initiates a non-contention random access procedure after downlink synchronization is completed;
Operation 310: The target eNB returns a Random Access Channel (RACH) Response message carrying an uplink resource, and a Timing Advance (TA) of the UE, allocated for the UE;
After the uplink synchronization is completed, the UE transmits and receives a user plane bearer and a control plane bearer using a new security key and the new PDCP, RLC and MAC entities;
Operation 311: The UE returns a handover completion message (i.e., an RRC reconfiguration complete message) to the target eNB; and thereafter the UE can transmit and receive data with the target eNB;
Correspondingly the target eNB returns an RLC Acknowledgement (ACK) message upon reception of the RRC reconfiguration complete message; and the UE starts to transmit uplink data of the user plane upon reception of the ACK message to the RRC reconfiguration complete message;
Operation 312: The target eNB initiates a Path Switch Request to an MME;
Operation 313: The MME initiates a Bearer Modify Request to an S-GW;
Operation 314: The S-GW switches the path;
Operation 315: The S-GW returns a Bearer Modify Response to the MME;
Operation 316: The MME returns a Path Switch Response to the target eNB; and thus the path has been switched;
Operation 317: The target eNB transmits a UE Context Release instruction to the source eNB; and
Operation 318: The source eNB releases the related resource allocated for the switched UE.
Since there are such a large number of local eNBs with small coverage that the UE may be handed over frequently between the cell corresponding to the macro eNB and the cell corresponding to the local eNB, a network deployment scenario where the user plane is separated from the control plane has been introduced in order to lower the frequency at which the UE is handed over, and at this time the UE is connected with both of the eNBs, and there are the following two network architectures proposed at present where bearers are separated:
In a first architecture as illustrated in FIG. 4, all of SRBs of the UE are maintained at the macro eNB, and all or a part of Data Radio Bearers (DRBs) are transferred to the local eNB for transmission, where an interface represented in dotted lines exist only if a part of the DRBs are separated.
In a second architecture as illustrated in FIG. 5, this architecture differs from the first architecture generally in that the local eNB can perform a part of RRC management functions (e.g., radio resource management or measurement, etc.), but RRC connections are still maintained at the macro eNB.
For the deployment of a HetNet, such a scenario may occur that a small cell is deployed in a macro coverage area of a plurality of macro eNBs, as illustrated in FIG. 6, where the small cell is located in an area in which two macro eNBs overlap, and in this scenario, the performance in the edge area of the macro eNB and the satisfaction of an edge user can be improved. The UE for which bearers are separated may need to be handed over from one of the macro eNB (referred to a source macro eNB) to the other macro eNB (referred to a target macro eNB), and then there has been absent in the existing protocol a solution to the UE performing handover between the macro eNBs in the bear separation scenario, particularly in the bear separation scenario where the local eNB or the small cell is shared by the plurality of macro eNBs.
In summary, there has been absent in the existing protocol a particular solution to performing handover of a UE between macro eNBs, and a particular solution to transmitting data in the handover process, in the bear separation scenario.